key_set identifies the rows to be yielded.
key_set names the
primary keys of the rows in table to be yielded, unless index is present.
If index is present, then key_set instead names index keys in index. If
the partition_token field is empty, rows are yielded in table primary key
order (if index is empty) or index key order (if index is non-empty). If
the partition_token field is not empty, rows will be yielded in an
unspecified order. It is not an error for the
key_set to name rows that
do not exist in the database. Read yields nothing for nonexistent rows.